This invention relates to a method for producing a porous cross-linked polymer material by polymerizing a water in oil type high internal phase emulsion (hereinafter referred to as xe2x80x9cHIPExe2x80x9d) containing a specific cross-linking agent, and more particularly to a method for producing a porous cross-linked polymer material excellent in absorbancy, mechanical strength, and flexibility by incorporating a compound having a double bond equivalent of not less than 120 g/mol.
For the production of a porous substance formed of uniform open cells having a microscopic diameter, it has been known to obtain a polymer in the HIPE in the presence of a specific surfactant has been known. The xe2x80x9chigh internal phase emulsionxe2x80x9d as used herein is generally construed as an emulsion containing a dispersion phase at a ratio exceeding 70 vol. % based on the total volume of the emulsion (K. J. Lissant, Journal of Colloid and Interface Science, Vol.22,462(1966)). U.S. Pat. No. 5,334,621, for example, discloses a method for producing a porous material by cross-link polymerizing a polymerizing monomer contained in such an HIPE (hereinafter referred to as xe2x80x9cHIPExe2x80x9d method).
This HIPE method produces a porous material by preparing an HIPE containing (i) a polymerizing monomer mixture containing an oil-soluble vinyl monomer and a cross-linking monomer having not less than two functional groups in the molecular unit, (ii) a water phase occupying 90 wt. %, preferably 95 wt. %, and particularly preferably 97 wt. %, of the emulsion, (iii) a surfactant such as a sorbitan fatty ester and a glycerol monofatty ester, and (iv) a polymerization initiator and heating the HIPE thereby polymerizing and cross-linking the HIPE. Generally, a porous cross-linked polymer is produced by mixing the oil phase containing at least the components (i) and (iii) mentioned above and the water phase of (ii), emulsifying the resultant mixture thereby preparing an HIPE, adding to the HIPE the initiator of (iv) and, at the same time, heating them to a temperature optimal for polymerization thereby initiating polymerization. According to this HIPE method, since the porous material formed of reticular open cells is produced by the reversed-phase emulsion polymerization, the produced porous material is possessed of characteristic properties as low density, absorbancy, water retaining property, heat insulating property, and sound insulating property.
The porous cross-linked polymer material of this quality is a highly bulky substance. For the purpose of producing, transporting, and storing this bulky material, it is required to possess ample flexibility enough to resume the original shape after it has been placed in a compressed or bent state for along time. When the porous cross-linked polymer material is used as an absorbent for humor, it demands excellent absorbancy.
An invention which, with a view to improving the properties of absorbancy and flexibility, has originated in the discovery that this improvement depends on the distribution of pore diameters in the HIPE has been disclosed in the official gazette of National Unexamined Patent Publication 10-521,187. This official gazette discloses a method for producing a porous cross-linked polymer material by using divinyl benzene and 1,6-hexadiol diacrylate as cross-linking monomers, emulsifying them in a specific procedure thereby obtaining an HIPE, and then polymerizing this HIPE at a temperature of 65xc2x0 C. for 18 hours.
The production of a macromolecular compound possessed of such a reticular texture from the HIPE is accomplished by the formation of a three-dimensionally cross-linked structure. As a technique for obtaining a low density porous cross-linked polymer material possessed of an effective degree of elasticity, the official gazette of JP-A-62-250,002, for example, discloses a method which is characterized by using a specific polymerizing monomer and fixing an average pore diameter in a specific range. This method obtains an elastic cross-linked porous polymer, for example, by combining up to 50 wt. % of styrene and at least 50 wt. % of an alkyl acrylate or methacrylate thereby forming an emulsion under controlled treating conditions, and polymerizing the monomer mixture.
The method disclosed in the official gazette of National Unexamined Patent Publication 10-512,187 as targeted at improving the quality of a porous cross-linked polymer material by controlling the distribution of pore diameters, however, fails to constitute a method to cure the HIPE in a short time because it requires several hours for polymerization and fails to complete a product in a short time.
When a porous cross-linked polymer material is produced by polymerizing the HIPE, the mechanical strength of the produced porous cross-linked polymer material is varied by difference of the polymerization time. Generally, the polymerization completed in a short time tends to degrade the mechanical strength of the produced porous cross-linking polymer material because the formation of a structure of the material is insufficient. Thus, the produced porous cross-linked polymer material possibly sustains a crack due to a break or a bend at the stage of fabrication and the stage for storage and transportation. If the polymerization time is simply elongated, the added time will result in a lowering the production efficiency. In this respect, by simply specifying the polymerizing monomer to be used as disclosed in the official gazette of JP-A-62-250,002, it is still difficult to obtain a porous cross-linked polymer material which secures ample absorbancy and excels in flexibility.
The present inventors have studied in detail the process for the production of a porous cross-linked polymer material by the HIPE method and have consequently discovered that a porous cross-linked polymer material excelling in mechanical strength and in absorbancy as well can be produced by using a specific compound as a cross-linking agent. This invention has been perfected as a result. Specifically, this invention is aimed at providing the following item (1).
(1) A method for the production of a porous cross-linked polymer material by the polymerization of an HIPE containing a cross-linking agent, characterized by at least one kind of said cross-linking agent being a compound having a double bond equivalent of not less than 120 g/mol.
According to this invention, a porous cross-linked polymer material excelling in absorbancy and flexibility can be produced by incorporating in the monomer mixture a compound having a double bond equivalent of not less than 120 g/mol. Moreover, by using the compound having a double bond equivalent of not less than 120 g/mol, it is made possible to adjust the cross-link density and advance the occurrence of the gel point and eventually curtail the polymerizing time. This fact, when the HIPE is continuously polymerized in the form of a sheet, contributes to the reduction of the space for production because the production time can be shortened and, at the same time, the apparatus for polymerization can be decreased in size. Since the produced porous cross-linked polymer material excels in flexibility, it sustains a crack only sparingly when it is subjected to such additional treatment as the impregnation in a chemical agent or when it is folded or bent during storage or transportation.